140 TRANSMISSION OF FACTORS 



the results of crossing the pink hybrid (RW) with the pure 

 white (WW) come about. The white form can only produce 

 gametes with the white factor but we have seen that the hybrid 

 produces gametes, one half of which contain the white factor and 

 one half the red factor. Therefore in the cross between the pink 

 hybrid and the pure white there are only two possible combina- 

 tions of gametes. A gamete from the hybrid bearing the white 

 factor may fuse with a gamete from the white four o'clock, giving 

 a pure white offspring or a gamete from the hybrid bearing the 

 red factor may fuse with a gamete from the white four o'clock, 

 giving a pink hybrid. 



It is now known that the hereditary substances or factors are 

 associated with the chromosomes and that they are separated 

 in a very definite way during the cell divisions that result in the 

 formation of the gametes. These chromosomes are associated 

 in pairs and in the division of the cell one member of a pair goes 

 to one daughter cell and the other member of this pair goes to 

 the other daughter cell. If now the factor for red is in one member 

 of a pair of chromosomes and the factor for white is in the other 

 member of this pair, it must follow in the above cell division that 

 one daughter cell will contain the red factor and the other 

 daughter cell will contain the white factor. Therefore the 

 gametes derived from these two daughter cells must contain 

 respectively the red factor and the white factor. This distri- 

 bution of the factors will be more manifest by examining the 

 accompanying diagram, Fig. 86 B. In figure I is shown a 

 nucleus with three pairs of chromosomes, one pair very small, 

 another somewhat larger and a third pair in which the factors for 

 red and white are indicated by the letters R and W. Of course 

 it is not to be supposed that any factor is visible to the eye. In 

 the second figure the two members of each pair of chromosomes 

 is seen drawing together preparatory to cell division. In the 

 third figure cell division is under way and the chromosomes still 

 in pairs have been drawn to the center of the spindle, and in the 

 fourth figure we see a member of each pair being pulled to the 

 opposite pole of the spindle, where they will be organized into 

 two daughter nuclei. The later appearance of these two daughter 



